1. Field of the Invention
The invention relates to a glass composition having optimized processing properties as are advantageous, in particular, for producing outer tubes for fluorescent lamps and the use of this glass for producing tubes and for lamp outer tubes.
2. Description of Related Art
A widespread type of fluorescent lamp operates according to the principle of low pressure gas discharge. Here, a small amount of mercury and a gas mixture containing noble gas are introduced into an evacuated hollow space. The mercury atoms are excited and then emit ultraviolet radiation (UV). This is converted into visible light by a phosphor coating which is usually applied to the inside of the container. The gas-filled container is preferably configured as an elongated, thin, cylindrical tube and its walls are made of glasses which are very transparent to visible light. Miniaturized fluorescent lamps having an external diameter of a few millimeters (mm) are frequently used for background illumination in screens and VDUs and other display elements. The phosphor coating mentioned is baked onto the interior surface of the glass tubes. Corresponding fluorescent lamps are therefore also referred to as luminophore lamps.
The temperatures reached in lamp production, in particular in the baking of the phosphor layer, can sometimes be up to 700 degrees Celsius (° C.). Such temperatures are usually significantly above the transition temperature of most glasses used for such purposes. The strength and stability of the glass therefore decrease.
The high temperatures required during baking of the phosphor layer are sometimes up to about 150° C. above the glass transition temperature (Tg) of the glasses usually used for this purpose. Due to the high baking temperature, the glass structure of the glasses used changes, which can lead to geometric changes and also to changes in the glass surface. When processing the glass tubes to give fluorescent lamps, these can change in length due to shrinkage, become crooked or glass tubes which are in contact stick to one another at the points of contact. All these undesirable effects are highly undesirable and troublesome in the generally highly automated processes for lamp production.